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Giant panda Mei Xiang is focusing her care on the larger of her newborn twins, and the smaller cub’s up-and-down weight is a concern, Washington’s National Zoo said on Tuesday.

Mei Xiang, a star tourist draw in the US capital, gave birth on Saturday after being artificially inseminated in April. Giant pandas are among the world’s most endangered species.

“Mei Xiang has not been a willing participant in the panda team’s efforts to switch the cubs since 2 p.m. yesterday afternoon. She has the larger cub in her possession,” the zoo said in a statement.

Mei Xiang, the female giant panda at the Smithsonian’s National Zoo in Washington.Reuters

The zoo staff is caring for the smaller cub and is trying to swap the cubs in Mei Xiang’s possession every four hours, it said.

The smaller cub’s behaviors are good but “the team is concerned about its fluctuating weight since the cub is now more than 48 hours old,” the statement said.

The two babies are still fur-less and about the size of a stick of butter. The smaller cub is being administered antibiotics and is urinating and defecating well, and there is no sign of respiratory trouble, the statement said.

Mei Xiang was artificially inseminated with sperm from Hui Hui, a panda in China, and from the National Zoo’s Tian Tian.

Zoo officials have said that they do not yet know which insemination was successful, and that it is possible the twins have different fathers.

Giant pandas, native to China, have a very low reproductive rate, especially in captivity. There are about 300 giant pandas in captivity and roughly 1,600 in the wild.